This invention relates generally to rotary earth drilling equipment of the type used to drill oil and gas wells and more particularly to an improved pipe elevator used to lift stands of pipe into and out of the well bore at substantial depths below the earth's surface.
At well depths up to about 15000 feet, the pipe string is commonly run into and out of the well bore by using a pipe elevator and slip combination. When running into the well bore, the pipe string is lowered by a single elevator and supported at the rotary table by gripping the pipe with slips that wedge in a slip bowl in a master bushing which in turn seats within the rotary table. The slips contain dies having sharp teeth which bite into the pipe. Once the slips are engaged, the elevator is released and hoisted up the derrick for connection to the next stand of pipe to be lowered. The elevator then supports the next stand of pipe while the pipe's lower end is connected to the pipe string in the well bore. After the ends are connected, the slips are released and the pipe string is again lowered into the well bore until the new pipe section is in position to be gripped by the slips. These operations are repeated until the desired length of pipe has been lowered into the well bore.
The operations are reversed to raise pipe from the well bore. The elevator is connected to the end of the drill string which protrudes from the bore and the pipe string is hoisted up the derrick. After raising a stand of pipe, the slips are set at the rotary table and the stand is uncoupled and carried by the elevator to the racking position. The elevator is then released and lowered to the rotary table to engage the protruding end of the following stand of pipe.
Although the above procedure is adequate for shallow depths, as the bore deepens the weight of the pipe string increases and places greater force on the dies in the rotary slips. The increased load causes the slips to striate or score the pipe which can result in costly damage. At times, migrating cracks caused by die scoring have brought about failure and parting of the pipe string.
To solve the problem of pipe striation, drillers have come to employ two conventional pipe elevators which are alternatively operated to support the string from the rotary table and handle the stand that is to be removed from or added to the drill string. The use of duel elevators, however, has required manual shifting of the elevators resulting in slower operating time and greater risk of injury to operating personnel. The problem was aggrevated by previous elevators which utilize pivoting arms secured by bolts for receiving and engaging the steel links which connect the elevator to the hook and traveling block. In order to remove the connecting links, the operator was required to manually fasten, unfasten and pivot the link arms.
One previously proposed elevator had spring operated link arms which cooperated with a specially designed track in the rotary table to automatically engage and disengage the steel links. In addition to requiring a modified rotary set up, this design had a link opening that overly restricted movement of the links resulting in binding between the links and the elevator body under certain conditions. The design also restricted the size of the return spring used to return the arms to the locked position and thereby increased the frequency of failure of the unit.